COMMUNICATING METHOD BETWEEN IPv4 TERMINAL AND IPv6 TERMINAL AND IPv4-IPv6 CONVERTING APPARATUS

ABSTRACT

In an apparatus, there are provided an IP transmitting and receiving unit for transmitting and receiving an IPv4 packet and an IPv6 packet; an IP header converting unit for performing a mutual conversion of the IPv4 packet and the IPv6 packet by an IP header conversion; a DNS substituting unit for receiving a domain information capturing request sent from an IPv4 terminal or an IPv6 terminal and substituting its process; an IPv4 address capturing unit for capturing an IPv4 address from a DHCP server; and an IP address conversion information holding unit for holding an IPv6 address of the IPv6 terminal and the IPv4 address captured by the IPv4 address capturing unit in correspondence to each other. Thus, a communication with the IPv4 terminal can be executed without preliminarily fixedly allocating the IPv4 address to the IPv6 terminal.

CROSS-REFERENCES

This is a continuation application of U.S. Ser. No. 12/764,661, filedApr. 21, 2010, which is a continuation application of U.S. Ser. No.11/699,429, filed Jan. 30, 2007 (now U.S. Pat. No. 7,746,863), which isa continuation application of U.S. Ser. No. 11/179,501, filed Jul. 13,2005 (now U.S. Pat. No. 7,193,998), which is a continuation applicationof U.S. Ser. No. 10/143,973, filed May 14, 2002 (now U.S. Pat. No.6,928,077), which is a continuation application of U.S. Ser. No.09/614,715, filed Jul. 12, 2000 (now U.S. Pat. No. 6,690,669), which isa continuation application of U.S. Ser. No. 08/959,462, filed Oct. 28,1997 (now U.S. Pat. No. 6,118,784). This application claims priority toJP 09-212889, filed Aug. 7, 1997 and JP 08-291480, filed Nov. 1, 1996.

BACKGROUND OF THE INVENTION

The present invention relates to a communicating method between an IPv4terminal and an IPv6 terminal and to an IPv4-IPv6 converting apparatus.More particularly, the invention relates to method and apparatus forrealizing a communication between an IPv4 terminal using IPv4 (InternetProtocol version 4) as a communication protocol and an IPv6 terminalusing IPv6 (Internet Protocol version 6) as a communication protocol.

As a method of realizing a communication between an IPv4 terminal and anIPv6 terminal, there is a method of using an ‘IPv4-mapped IPv6 address’and an ‘IPv4-compatible IPv6 address’ described in RFC (Request ForComments) 1884 issued by IETF (Internet Engineering Task Force).

The ‘IPv4-mapped IPv6 address’ is an IPv6 address in which “0” is set tothe 127th to 48th bits among 128 bits of the IPv6 address, “1” is set tothe 47th to 32nd bits, and an ‘IPv4 address’ is set to the 31st to 0thbits.

The ‘IPv4-compatible IPv6 address’ is an IPv6 address in which “0” isset to the 127th to 32nd bits among 128 bits of the IPv6 address and the‘IPv4 address’ is set to the 31st to 0th bits.

When the communication between the IPv4 terminal and the IPv6 terminalis executed, an. IPv4 address is preliminarily fixedly allocated to theIPv6 terminal which communicates with the IPv4 terminal. A packetconverting apparatus is provided on the way of a path through which theIPv4 terminal and the IPv6 terminal communicate and a mutual conversionof IP headers of an IPv4 packet (RFC791) and an IPv6 packet (RFC1883) isperformed.

In an IPv4 network, the IPv4 address of the IPv4 terminal and the IPv4address allocated to the IPv6 terminal are used and a communication isexecuted by using the IPv4 packet. In an IPv6 network, the ‘IPv4mappedIPv6 address’ in which the ‘IPv4 address of the IPv4 terminal’ has beenset is used as an IPv6 address of the IPv4 terminal. In the IPv6network, the ‘IPv4compatible IPv6 address’ in which the ‘IPv4 addressallocated to the IPv6 terminal’ has been set is used as an IP address ofthe IPv6 terminal and a communication is executed by using the IPv6packet.

The packet converting apparatus converts the ‘IPv4-mapped IPv6 address’included in the IPv6 packet into the ‘IPv4 address of the IPv4 terminal’and converts the ‘IPv4-compatible IPv6 address’ to the ‘IPv4 addressallocated to the IPv6 terminal’, thereby converting the IPv6 packet intothe IPv4 packet. Contrarily, the ‘IPv4 address of the IPv4 terminal’included in the IPv4 packet is converted into the ‘IPv4-mapped IPv6address’ and the ‘IPv4 address allocated to the IPv6 terminal’ isconverted into the ‘IPv4-compatible IPv6 address’, thereby convertingthe IPv4 packet into the IPv6 packet. Consequently, the communicationbetween the IPv4 terminal and the IPv6 terminal can be executed.

As a method of realizing the communication between the IPv4 terminal andthe IPv6 terminal, IETF also proposed a method called a dual stack.According to the method, the IPv6 terminal has both of the communicationprotocols of IPv4 and IPv6. A communication is executed between the IPv6terminals by using the IPv6 protocol. A communication is executedbetween the IPv4 terminals by using the IPv4 protocol. Thus, thecommunication between the IPv4 terminal and the IPv6 terminal can beexecuted.

IETF also proposed a method called an IP tunneling. This is a methodsuch that when the IPv4 network exists on a communication path betweentwo IPv6 terminals and a communication cannot be executed directly bythe IPv6 packet, the IPv6 packet is encapsulated by the IPv4 header andis allowed to pass through the IPv4 network. Similarly, when the IPv6network exists on a communication path between the IPv4 terminals and acommunication cannot be executed directly by the IPv4 packet, the IPv4packet is encapsulated by the IPv6 header and is allowed to pass throughthe IPv6 network. Consequently, even when there is the IPv4 network onthe communication path, the communication between the IPv6 terminals canbe executed. Even when there is the IPv6 network on the communicationpath, the communication between the IPv4 terminals can be executed.

According to the method, the communication between the IPv4 terminal andthe IPv6 terminal can be realized by a simple operation of adding afixed pattern of 96 bits to the IPv4 address or deleting the fixedpattern of 96 bits from the IPv6 address.

According to the method called a dual stack, by selectively using thecommunication protocols of IPv4 and IPv6 in accordance with acommunication partner, the communication between the IPv4 terminal andthe IPv6 terminal can be realized.

According to the method called an IP tunneling, by encapsulating thepacket by the header of the relevant communication protocol and passingthe resultant data through the network existing on the communicationpath between the two terminals, the communication between the twoterminals can be realized.

SUMMARY OF THE INVENTION

The convention techniques, however, have the following problems.

A first problem is that the IPv4 address has to be fixedly allocated toall of the IPv6 terminals which have a possibility of communication withthe IPv4 terminal. This further consumes the IPv4 address which is shortand accelerates its shortage.

A second problem is that since mutual domain information (for example,an IP address corresponding to a domain name) cannot be referred tobetween the IPv4 network and the IPv6 network, the partner can bedesignated not by the domain name but by only an actual IP address. Thisresults in that the communication partners have to be mutually awarewhether the partner uses IPv4 or IPv6 as a communication protocol.

A third problem is that, in the method called a dual stack, the IPv4protocol is provided for all of the IPv6 terminals having thepossibility of communication with the IPv4 terminal and the IPv4 addresshas to be fixedly allocated. This results in that a larger memorycapacity is necessary as compared with that in case of providing onlythe IPv6 protocol and, in a manner similar to the first problem, theIPv4 address which is short is further consumed and the shortage isaccelerated.

A fourth problem is that, in the method called an IP tunneling, when theIPv4 network exists on the communication path between the two IPv6terminals, in order to pass the IPv6 packet by encapsulating it by theIPv4 header, a setting for it has to be preliminarily performed.Similarly, when the IPv6 network exists on the communication pathbetween the two IPv4 terminals, in order to pass the IPv4 packet byencapsulating it by the IPv6 header, a setting for it has to bepreliminarily performed. The method can be used only between theterminals in which the setting of the IP tunneling has preliminarilybeen performed. The communication cannot be executed between theterminals in which the setting is not preliminarily performed.

It is, therefore, an object of the invention to provide method andapparatus in which only an IPv6 protocol is provided for an IPv6terminal, even if an IPv4 address is not preliminarily fixedlyallocated, a communication between an IPv4 terminal and the IPv6terminal can be executed, a partner is designated not by an IP addressbut by a domain name, and the communication between the IPv4 terminaland the IPv6 terminal can be executed.

According to the first aspect, in a communication network systemcomprising an IPv4 network, an IPv6 networks and an IPv4-IPv6 convertingapparatus connected to both of them, an IPv4 terminal of the IPv4network inquires an IP address for a domain name of an IPv6 terminal ofthe IPv6 network to the IPv4-IPv6 converting apparatus. Thus, theIPv4-IPv6 converting apparatus obtains the IPv6 address corresponding tothe domain name from a DNS (Domain Name System) v6 server apparatus formanaging domain information of the IPv6 network, dynamically acquiresthe IPv4 address corresponding to the IPv6 address from a DHCP (DynamicHost Configuration Protocol) v4 server apparatus for dynamicallyallocating the IPv4 address, and notifies to the IPv4 terminal. The IPv4terminal forms an IPv4 packet by setting an own IPv4 address to an IPv4source address and by setting the notified IPv4 address to an IPv4destination address and sends the IPv4 packet to the IPv4-IPv6converting apparatus. The IPv4-IPv6 converting apparatus converts theIPv4 source address included in the received IPv4 packet into an IPv6source address by adding fixed data to the IPv4 source address andconverts an IPv4 destination address included in the IPv4 packet intothe IPv6 address corresponding to the IPv4 destination address, therebyobtaining an IPv6 destination address, forming an IPv6 packet from theIPv4 packet, and sending the

IPv6 packet to the IPv6 network. The IPv6 terminal forms the IPv6 packetby setting the own IPv6 address to the IPv6 source address and settingthe IPv6 source address included in the received IPv6 packet to the IPv6destination address, and transmits the IPv6 packet to the IPv4-IPv6converting apparatus. The IPv4-IPv6 converting apparatus deletes thefixed data from the IPv6 destination address included in the receivedIPv6 packet, converts the resultant IPv6 destination address to the IPv4destination address, and converts the IPv6 source address included inthe IPv6 packet to the IPv4 address corresponding to the IPv6 sourceaddress, thereby obtaining the IPv4 source address, forming the IPv4packet from the IPv6 packet, and transmitting the IPv4 packet to theIPv4 network.

In the communicating method between the IPv4 terminal and the IPv6terminal according to the first aspect, when the communication from theIPv4 terminal side of the IPv4 network to the IPv6 terminal of the IPv6network is started, the IPv4 address is dynamically allocated to theIPv6 terminal of the IPv6 network. Consequently, it is unnecessary topreliminarily fixedly allocate the IPv4 address to the IPv6 terminal, sothat the consumption of the IPv4 address can be suppressed. Since theIPv4-IPv6 converting apparatus automatically obtains the IP address forthe domain name of the IPv6 terminal from the DNSv6 server apparatus,the IPv4 terminal can designate the domain name and communicate with theIPv6 terminal.

According to the second aspect, in a communication network systemcomprising an IPv4 network, an IPv6 network, and an IPv4-IPv6 convertingapparatus connected to both of them, an IPv6 terminal of the IPv6network inquires an IP address for a domain name of an IPv4 terminal ofthe IPv4 network to the IPv4-IPv6 converting apparatus. Thus, theIPv4-IPv6 converting apparatus acquires an IPv4 address corresponding tothe domain name from a DNSv4 server apparatus for managing domaininformation of the IPv4 network, converts the IPv4 address to the IPv6address by adding fixed data to the IPv4 address, and notifies the IPv6terminal of the IPv6 address. The IPv6 terminal forms an IPv6 packet bysetting an own IPv6 address to an IPv6 source address and setting thenotified IPv6 address to an IPv6 destination address and transmits theIPv6 packet to the IPv4-IPv6 converting apparatus. The IPv4-IPv6converting apparatus deletes the fixed data from the IPv6 destinationaddress included in the received IPv6 packet and converts the resultantIPv6 destination address to an IPv4 destination address, dynamicallyacquires the IPv4 address corresponding to the IPv6 source addressincluded in the IPv6 packet from a DHCPv4 server apparatus fordynamically allocating the IPv4 address, forms the IPv4 packet from theIPv6 packet by using the IPv4 address, and transmits the IPv4 packet tothe IPv4 network. The IPv4 terminal forms the IPv4 packet by setting theown IPv4 address to an IPv4 source address and setting the IPv4 sourceaddress included in the received IPv4 packet to an IPv4 destinationaddress and transmits the IPv4 packet to the IPv4-IPv6 convertingapparatus. The IPv4-IPv6 converting apparatus converts the IPv4 sourceaddress to the IPv6 source address by adding fixed data to the IPv4source address, obtains the IPv6 destination address by converting theIPv4 destination address included in the IPv4 packet to the IPv6 addresscorresponding to the IPv4 destination address, forms the IPv6 packetfrom the IPv4 packet, and transmits the IPv6 packet to the IPv6 network.

In the communicating method between the IPv4 terminal and the IPv6terminal according to the second aspect, when the communication from theIPv6 terminal side of the IPv6 network to the IPv4 terminal of the IPv4network is started, the IPv4 address is dynamically allocated to theIPv6 terminal of the IPv6 network. Consequently, it is unnecessary topreliminarily fixedly allocate the IPv4 address to the IPv6 terminal, sothat the consumption of the IPv4 address can be suppressed. Since theIPv4-IPv6 converting apparatus automatically obtains the IP address forthe domain name of the IPv4 terminal from the DNSv4 server apparatus,the IPv6 terminal can designate the domain name and communicate with theIPv4 terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a constructional diagram of an IPv4-IPv6 converting apparatusaccording to the first embodiment of the invention;

FIG. 2 is a constructional diagram of an IP address conversion table;

FIG. 3 is a constructional diagram of a communication network systemaccording to the first embodiment of the invention;

FIG. 4 is a flowchart in case of starting a communication from an IPv4terminal to an IPv6 terminal through the IPv4-IPv6 converting apparatusaccording to the first embodiment of the invention;

FIG. 5 is a flowchart which is continuous to FIG. 4;

FIG. 6 is a flowchart in case of starting a communication from the IPv6terminal to the IPv4 terminal through the IPv4-IPv6 converting apparatusaccording to the first embodiment of the invention;

FIG. 7 is a flowchart which is continuous to FIG. 6;

FIG. 8 is a flowchart in another case of starting a communication fromthe IPv6 terminal to the IPv4 terminal;

FIG. 9 is a flowchart which is continuous to FIG. 8;

FIG. 10 is a constructional diagram of another IPv4-IPv6 convertingapparatus;

FIG. 11 is a constructional diagram of another network system;

FIG. 12 is a flowchart in another case of starting a communication fromthe IPv4 terminal to the IPv6 terminal;

FIG. 13 is a flowchart which is continuous to FIG. 12;

FIG. 14 is a flowchart in still another case of starting a communicationfrom the IPv6 terminal to the IPv4 terminal;

FIG. 15 is a flowchart which is continuous to FIG. 14;

FIG. 16 is a constructional diagram of an IPv4-IPv6 converting apparatusaccording to the second embodiment of the invention;

FIG. 17 is a constructional diagram of an IPv4-IPv6 converting apparatusaccording to the third embodiment of the invention;

FIG. 18 is a constructional diagram of an IPv4-IPv6 converting apparatusaccording to the fourth embodiment of the invention;

FIG. 19 is a constructional diagram of a communication network systemaccording to the fifth embodiment of the invention;

FIG. 20 is a flowchart in case of communicating between the IPv6terminals through the IPv4-IPv6 converting apparatus according to thefifth embodiment of the invention; and

FIG. 21 is a flowchart which is continuous to FIG. 20.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described hereinbelowwith reference to the drawings.

(A) First Embodiment

FIG. 1 is a constructional diagram of an IPv4-IPv6 converting apparatus1 a according to the first embodiment of the invention.

The IPv4-IPv6 converting apparatus 1 a is located between an IPv4network and an IPv6 network and realizes a communication between an IPv4terminal and an IPv6 terminal by performing an IP header conversion ofan IPv4 packet and an IPv6 packet. The IPv4-IPv6 converting apparatus 1a comprises IP transmitting and receiving means 10, IP header convertingmeans 11, DNS substituting means 13, IPv4 address capturing means 14,and IP address conversion information holding means 15.

The IP transmitting and receiving means 10 is means for transmitting andreceiving the IPv4 packet and IPv6 packet between the IPv4 network andthe IPv6 network and is constructed by, for example, electronic devicessuch as CPU, RAM, and the like.

The IP header converting means 11 is means for performing an IP headerconversion of the IPv4 packet and the IPv6 packet and is constructed by,for example, electronic devices such as CPU, RAM, and the like.

The DNS substituting means 13 is means for capturing domain informationfrom a DNS server in the IPv4 network or a DNS server in the IPv6network in accordance with the DNS technique disclosed in RFC1034,RFC1886, or the like and is constructed by, for example, electronicdevices such as CPU, RAM, and the like.

The IPv4 address capturing means 14 is means for capturing an IPv4address from a DHCP server in the IPv4 network in accordance with theDHCP technique disclosed in RFC1541 or the like and is constructed by,for example, electronic devices such as CPU, RAM, and the like.

The IP address conversion information holding means 15 is means forconverting the IPv4 address and the IPv6 address and is constructed by,for example, electronic devices such as an RAM and the like. Acorrespondence between the IPv4 address and the IPv6 address is held asan IP address conversion table 151.

FIG. 2 is a constructional diagram of the IP address conversion table151. IPv6 addresses of the IPv6 terminal communicating with the IPv4terminal and IPv4 addresses which are dynamically allocated incorrespondence to them have been registered in the IP address conversiontable 151.

FIG. 3 is a diagram of a communication network system 100 connecting anIPv4 network 104 and an IPv6 network 106 through the IPv4-IPv6converting apparatus 1 a.

In addition to an IPv4 terminal 2, a DNSv4 server 3 for managing domaininformation in the IPv4 network 104 and a DHCPv4 server 4 fordynamically allocating the IPv4 addresses to terminals in the IPv4network 104 are connected to the IPv4 network 104.

In addition to an IPv6 terminal 5, a DNSv6 server 6 for managing domaininformation in the IPv6 network is connected to the IPv6 network 106.

It is now assumed that “133.144.95.101” has been allocated as an IPv4address to the IPv4 terminal 2 and “1::1” has been allocated as an IPv6address to the IPv6 terminal 5. It is also assumed that 100 IPv4addresses of “133.144.95.1” to “133.144.95.100” have been pooled in theDHCPv4 server 4. It is also assumed that the IPv4-IPv6 convertingapparatus 1 a has been set as a DNS server into the IPv4 terminal 2 andIPv6 terminal 5. Further, it is assumed that the DNSv4 server 3 andDNSv6 server 6 have been set as DNS servers and the DHCPv4 server 4 hasbeen set as a DHCP server into the IPv4-IPv6 converting apparatus 1 a.

In FIG. 3, although the IPv4 network 104 and IPv6 network 106 areperfectly separated, a similar construction is also applied to a casewhere the IPv4 network and IPv6 network physically mixedly exist in thesame network.

FIGS. 4 and 5 are flowcharts of the operation in case of starting acommunication from the IPv4 terminal 2 to the IPv6 terminal 5.

Although the IPv4 terminal 2 knows a domain name of the IPv6 terminal 5,it doesn't know an IP address. As shown in FIG. 4, therefore, after theIPv4 terminal 2 transmitted an inquiry message (hereinafter, called a‘message A’) of the IP address corresponding to the domain name of theIPv6 terminal 5 to the IPv4-IPv6 converting apparatus 1 a via the IPv4network 104, it waits for a response for a predetermined time.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 a receives and processes the message A and sends it to theDNS substituting means 13. The DNS substituting means 13 instructs theIP transmitting and receiving means 10 so as to transfer the message Ato the DNSv4 server 3, sends the message A thereto, and after that,waits for a response for a predetermined time. The IP transmitting andreceiving means 10 transfers the message A to the DNSv4 server 3 via theIPv4 network.

The DNSv4 server 3 which received the message A retrieves the domaininformation of the IPv6 terminal 5. When it is not registered, however,a response to the message A is not returned.

The DNS substituting means 13 which didn't receive the response from theDNSv4 server 3 even after the elapse of a predetermined time instructsthe IP transmitting and receiving means 10 so as to transfer the messageA to the DNSv6 server 6, sends the message A thereto, and after that,waits for a response for a predetermined time. The IP transmitting andreceiving means 10 transfers the message A to the DNSv6 server 6 via theIPv6 network.

The DNSv6 server 6 which received the message A retrieves the domaininformation of the IPv6 terminal 5 and returns the registered domaininformation of the IPv6 terminal 5 to the IPv4-IPv6 converting apparatus1 a as a response message (hereinafter, referred to as a ‘message B’)for the message A.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 a which received the response from the DNSv6 server 6 withinthe predetermined time sends the received message B to the DNSsubstituting means 13. The DNS substituting means 13 obtains the IPv6address (1::1) corresponding to the domain name of the IPv6 terminal 5from the message B and sends it to the IP address conversion informationholding means 15. The IP address conversion information holding means 15retrieves the IP address conversion table 151 by using the IPv6 address(1::1) of the IPv6 terminal 5 as a key. If the relevant entry has beenregistered, the IPv4 address of the entry is returned to the DNSsubstituting means 13. On the other hand, when the relevant entry is notregistered yet, such a fact is notified to the DNS substituting means13.

The DNS substituting means 13 which received the IPv4 address from theIP address conversion information holding means 15 forms a message(hereinafter, referred to as a ‘message E’) to notify of the IPv4address as an IPv4 address corresponding to the domain name of the IPv6terminal 5 and sends it to the IP transmitting and receiving means 10.The IP transmitting and receiving means 10 transmits the message E tothe IPv4 terminal 2 via the IPv4 network.

On the other hand, the DNS substituting means 13 which received thenotification indicating that the entry is not registered yet from the IPaddress conversion information holding means 15 instructs the IPv4address capturing means 14 so as to capture the IPv4 address and, afterthat, waits for a response for a predetermined time. The IPv4 addresscapturing means 14 forms an IPv4 address capturing message (hereinafter,called a ‘message C’), instructs the IP transmitting and receiving means10 so as to transfer the message C to the DHCPv4 server 4, sends themessage C thereto, and after that, waits for a response for apredetermined time. The IP transmitting and receiving means 10 transmitsthe message C to the DHCPv4 server 4 via the IPv4 network.

The DHCPv4 server 4 which received the message C gives the IPv4 addressto the IPv6 address (1::1) of the IPv6 terminal 5 (it is now assumedthat “133.144.95.1” has been given) and returns an IPv4 address givenmessage (hereinafter, called a ‘message D’).

The IP transmitting and receiving means 10 which received the message Dsends the message D to the IPv4 address capturing means 14. The IPv4address capturing means 14 obtains the IPv4 address (133.144.95.1) fromthe message D and notifies the DNS substituting means 13. The DNSsubstituting means 13 forms the message E to notify of the IPv4 address(133.144.95.1) as an IP address corresponding to the domain name of theIPv6 terminal 5 and sends it to the IP transmitting and receiving means10. The IP transmitting and receiving means 10 transmits the message Eto the IPv4 terminal 2 via the IPv4 network. The DNS substituting means13 sends the IPv4 address (133.144.95.1) captured from the DHCPv4 server4 to the IP address conversion information holding means 15 incorrespondence to the IPv6 address (1::1) of the IPv6 terminal 5. The IPaddress conversion information holding means 15 forms a new entry intothe IP address conversion table 151 and registers a correspondence ofthe IPv4 address (133.144.95.1) and the IPv6 address (1::1) of the IPv6terminal 5. The entry of the IP address conversion table 151 is deletedin the case where it is not referred for a predetermined time. The IPv4address (133.144.95.1) is returned to the DHCPv4 server 4.

As shown in FIG. 5, the IPv4 terminal 2 transmits the IPv4 packet inwhich the IPv4 address (133.144.95.1) notified from the IPv4-IPv6converting apparatus 1 a by the message E has been set into an IPv4destination address field of the IPv4 packet and an own IPv4 address(133.144.95.101) has been set into an IPv4 source address field to theIPv4-IPv6 converting apparatus 1 a through the IPv4 network.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 a sends the received IPv4 packet to the IP header convertingmeans 11. The IP header converting means 11 extracts the IPv4 address(133.144.95.101) from the IPv4 source address field of the IPv4 packet,converts the IPv4 address into an ‘IPv4-mapped IPv6 address’(::FFFF:133.144.95.101) of 128 bits by adding a fixed pattern of 96 bitsmentioned above to the IPv4 address, and sets it into an IPv6 sourceaddress field of the IPv6 packet. The IP header converting means 11extracts the IPv4 address (133.144.95.1) from the IPv4 destinationaddress field of the IPv4 packet and sends it to the IP addressconversion information holding means 15. The IP address conversioninformation holding means 15 obtains the IPv6 address (1::1)corresponding to the IPv4 address (133.144.95.1) with reference to theIP address conversion table 151 and sends it to the IP header convertingmeans 11. The IP header converting means 11 sets the IPv6 address (1::1)into an IPv6 destination address field of the IPv6 packet. The IPv6packet formed by performing the IP header conversion as mentioned aboveis sent to the IP transmitting and receiving means 10. The IPtransmitting and receiving means 10 transmits the IPv6 packet to theIPv6 terminal 5 through the IPv6 network.

In this manner, the communication from the IPv4 terminal 2 to the IPv6terminal 5 can be started.

FIGS. 6 and 7 are flowcharts for the operation in case of starting acommunication from the IPv6 terminal 5 to the IPv4 terminal 2.

Although the IPv6 terminal 5 knows a domain name of the IPv4 terminal 2,it doesn't know the IP address. As shown in FIG. 6, therefore, the IPv6terminal 5 transmits an inquiry message (hereinafter, called a ‘messageK’) of an IP address corresponding to the domain name of the IPv4terminal 2 to the IPv4-IPv6 converting apparatus 1 a via the IPv6network 106 and, after that, waits for a response for a predeterminedtime.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 a receives and processes the message K and sends it to theDNS substituting means 13. The DNS substituting means 13 instructs theIP transmitting and receiving means 10 so as to transfer the message Kto the DNSv6 server 6, sends the message K thereto, and after that,waits for a response for a predetermined time. The IP transmitting andreceiving means 10 transfers the message K to the DNSv6 server 6 via theIPv6 network.

The DNSv6 server 6 which received the message K retrieves the domaininformation of the IPv4 terminal 2. However, when it is not registered,a response to the message K is not returned.

The DNS substituting means 13 which didn't receive the response from theDNSv6 server 6 even after the elapse of a predetermined time instructsthe IP transmitting and receiving means 10 so as to transfer the messageK to the DNSv4 server 3, sends the message K thereto, and after that,waits for a response for a predetermined time. The IP transmitting andreceiving means 10 transfers the message K to the DNSv4 server 3 via theIPv4 network.

The DNSv4 server 3 which received the message K retrieves the domaininformation of the IPv4 terminal 2 and returns the registered domaininformation of the IPv4 terminal 2 to the IPv4-IPv6 converting apparatus1 a as a response message (hereinafter, called a ‘message L’) for themessage K.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 a which received the response from the DNSv4 server 3 withinthe predetermined time sends the received message L to the DNSsubstituting means 13. The DNS substituting means 13 obtains the IPv4address (133.144.95.101) corresponding to the domain name of the IPv4terminal 2 from the message L.

Subsequently, the DNS substituting means 13 sends the IPv6 address(1::1) of the IPv6 terminal 5 to the IP address conversion informationholding means 15. The IP address conversion information holding means 15retrieves the IP address conversion table 151 by using the IPv6 address(1::1) of the IPv6 terminal 5 as a key and notifies the DNS substitutingmeans 13 of information about whether the relevant entry has beenregistered or not.

The DNS substituting means 13 which received a notification indicatingthat the relevant entry is not registered yet in the IPv6 address (1::1)of the IPv6 terminal 5 instructs the IPv4 address capturing means 14 soas to capture the IPv4 address and, after that, waits for a response fora predetermined time. The IPv4 address capturing means 14 forms an IPv4address capture message (hereinafter, called a ‘message M’), instructsthe IP transmitting and receiving means 10 so as to transfer the messageM to the DHCPv4 server 4, sends the message M thereto, and after thatwaits for a response for a predetermined time. The IP transmitting andreceiving means 10 transmits the message M to the DHCPv4 server 4 viathe IPv4 network.

The DHCPv4 server 4 which received the message M gives the IPv4 addressto the IPv6 address (1::1) of the IPv6 terminal 5 (it is now assumedthat “133.144.95.1” is given) and returns an IPv4 address given message(hereinafter, called a ‘message N’).

The IP transmitting and receiving means 10 which received the message Nsends the message N to the IPv4 address capturing means 14. The IPv4address capturing means 14 obtains the IPv4 address (133.144.95.1) fromthe message N and notifies the DNS substituting means 13. The DNSsubstituting means 13 sends the IPv4 address (133.144.95.1) capturedfrom the DHCPv4 server 4 to the IP address conversion informationholding means 15 in correspondence to the IPv6 address (1::1) of theIPv6 terminal 5. The IP address conversion information holding means 15forms a new entry into the IP address conversion table 151 and registersa correspondence of the IPv4 address (133.144.95.1) and the IPv6 address(1::1) of the IPv6 terminal 5. The entry in the IP address conversiontable 151 is deleted when it is not referred to for a predetermined timeand the IPv4 address (133.144.95.1) is returned to the DHCPv4 server 4.

When a notification indicating that the relevant entry has beenregistered in the IPv6 address (1::1) of the IPv6 terminal 5 is receivedor when the registration of the relevant entry into the IPv6 address(1::1) of the IPv6 terminal 5 is completed, the DNS substituting means13 forms a message (hereinafter, called a ‘message P’) to notify of an‘IPv4-mapped IPv6 address’ (::FFFF:133.144.95.101) in which the IPv4address (133.144.95.101) of the IPv4 terminal 2 has been set as an IPaddress corresponding to the domain name of the IPv4 terminal 2. The DNSsubstituting means 13 instructs the IP transmitting and receiving means10 so as to transfer the message P to the IPv6 terminal 5 and sends themessage P thereto. The IP transmitting and receiving means 10 transmitsthe message P to the IPv6 terminal 5 via the IPv6 network.

As shown in FIG. 7, the IPv6 terminal 5 transmits the IPv6 packet inwhich the IPv6 address (::FFFF:133.144.95.101) notified from theIPv4-IPv6 converting apparatus 1 a has been set into the IPv6destination address field of the IPv6 packet and the own IPv6 address(1::1) has been set into the IPv6 source address field to the IPv4-IPv6converting apparatus 1 a through the IPv6 network.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 a sends the received IPv6 packet to the IP header convertingmeans 11. The IP header converting means 11 extracts the IPv6 address(::FFFF:133.144.95.101) from the IPv6 destination address field of theIPv6 packet, converts the IPv6 address into the IPv4 address(133.144.95.101) of 32 bits by deleting the fixed pattern of 96 bitsmentioned above, and sets the IPv4 address into the IPv4 destinationaddress field of the IPv4 packet. The IP header converting means 11extracts the IPv6 address (1::1) from the IPv6 source address field ofthe IPv6 packet and sends it to the IP address conversion informationholding means 15. The IP address conversion information holding means 15obtains the IPv4 address (133.144.95.1) corresponding to the IPv6address (1::1) with reference to the IP address conversion table 151 andsends it to the IP header converting means 11. The IP header convertingmeans 11 sets the IPv4 address (133.144.95.1) into the IPv4 sourceaddress field of the IPv4 packet. The IPv4 packet formed by performingthe IP header conversion as mentioned above is sent to the IPtransmitting and receiving means 10. The IP transmitting and receivingmeans 10 transmits the IPv4 packet to the IPv4 terminal 2 through theIPv4 network.

In this manner, the communication from the IPv6 terminal 5 to the IPv4terminal 2 can be started.

Although the case where the IPv4 domain information had been registeredin the DNSv4 server 3 has been described above, the IPv4 domaininformation can be also registered in the DNSv6 server 6. In this case,the IPv4-IPv6 converting apparatus 1 a captures the IPv4 domaininformation by communicating with the DNSv6 server 6 by using the IPv6protocol. Similarly, the IPv6 domain information can be also registeredin the DNSv4 server 3. In this case, the IPv4-IPv6 converting apparatus1 a captures the IPv6 domain information by communicating with the DNSv4server 3 by using the IPv4 protocol.

Explanation has been made with respect to the case where the addressconversion is performed solely in the address field by adding ordeleting the fixed pattern of 96 bits mentioned above or by referring tothe IP address conversion table 151. As for the address on the terminalside which started the communication, however, a combination of theaddress on the terminal side which started the communication and theport number of a TCP (Transmission Control Protocol)/UDP (User DatagramProtocol) can be also converted to a combination of an address of theIPv4-IPv6 converting apparatus and a port number of the TCP/UDP by usinga method called an IP masquerade.

FIGS. 8 and 9 are flowcharts for operation in another case of starting acommunication from the IPv6 terminal 5 to the IPv4 terminal 2. This casehas a feature in that temporary allocation of IPv4 address to the IPv6terminal is performed at the time of actually starting a datacommunication.

Although the IPv6 terminal 5 knows a domain name of the IPv4 terminal 2,it doesn't know the IP address. As shown in FIG. 8, therefore, the IPv6terminal 5 transmits an inquiry message (hereinafter, called a ‘messageK’) of an IP address corresponding to the domain name of the IPv4terminal 2 to the IPv4-IPv6 converting apparatus 1 a via the IPv6network 106 and, after that, waits for a response for a predeterminedtime.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 a receives and processes the message K and sends it to theDNS substituting means 13. The DNS substituting means 13 instructs theIP transmitting and receiving means 10 so as to transfer the message Kto the DNSv6 server 6, sends the message K thereto, and after that,waits for a response for a predetermined time. The IP transmitting andreceiving means 10 transfers the message K to the DNSv6 server 6 via theIPv6 network.

The DNSv6 server 6 which received the message K retrieves the domaininformation of the IPv4 terminal 2. However, when it is not registered,a response to the message K is not returned.

The DNS substituting means 13 which didn't receive the response from theDNSv6 server 6 even after the elapse of a predetermined time instructsthe IP transmitting and receiving means 10 so as to transfer the messageK to the DNSv4 server 3, sends the message K thereto, and after that,waits for a response for a predetermined time. The IP transmitting andreceiving means 10 transfers the message K to the DNSv4 server 3 via theIPv4 network.

The DNSv4 server 3 which received the message K retrieves the domaininformation of the Ipv4 terminal 2 and returns the registered domaininformation of the IPv4 terminal 2 to the IPv4-IPv6 converting apparatus1 a as a response message (hereinafter, called a ‘message L’) for themessage K.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 a which received the response from the DNSv4 server 3 withinthe predetermined time sends the received message L to the DNSsubstituting means 13. The DNS substituting means 13 obtains the IPv4address (133.144.95.101) corresponding to the domain name of the IPv4terminal 2 from the message L.

The DNS substituting means 13 forms a message (hereinafter, called a‘message P’) to notify of an ‘IPv4-mapped IPv6 address’(::FFFF:133.144.95.101) in which the IPv4 address (133.144.95.101) ofthe IPv4 terminal 2 has been set as an IP address corresponding to thedomain name of the IPv4 terminal 2. The DNS substituting means 13instructs the IP transmitting and receiving means 10 so as to transferthe message P to the IPv6 terminal 5 and sends the message P thereto.The IP transmitting and receiving means 10 transmits the message P tothe IPv6 terminal 5 via the IPv6 network.

As shown in FIG. 9, the IPv6 terminal 5 transmits the IPv6 packet inwhich the IPv6 address (::FFFF:133.144.95.101) notified from theIPv4-IPv6 converting apparatus 1 a has been set into the IPv6destination address field of the IPv6 packet and the own IPv6 address(1::1) has been set into the IPv6 source address field to the IPv4-Ipv6converting apparatus 1 a through the IPv6 network.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 a sends the received IPv6 packet to the IP header convertingmeans 11. The IP header converting means 11 extracts the IPv6 address(::FFFF:133.144.95.101) from the Ipv6 destination address field of theIPv6 packet, converts the IPv6 address into the IPv4 address(133.144.95.101) of 32 bits by deleting the fixed pattern of 96 bitsmentioned above, and sets the IPv4 address into the IPv4 destinationaddress field of the IPv4 packet. The IP header converting means 11extracts the IPv6 address (1::1) from the IPv6 source address field ofthe IPv6 packet and sends it to the IP address conversion informationholding means 15.

The IP address conversion information holding means 15 retrieves the IPaddress conversion table 151 by using the IPv6 address (1::1) of theIPv6 terminal 5 as a key and notifies the IP header converting means 11of a result indicating whether the relevant entry has been registered ornot.

The IP header converting means 11 which received the notification thatthe relevant entry is not registered yet in the IPv6 address (1::1) ofthe IPv6 terminal 5 instructs the IPv4 address capturing means 14 so asto capture the IPv4 address and, after that, waits for a reply for apredetermined time. The IPv4 address capturing means 14 forms an IPv4address capturing message (hereinafter, referred to as a “message M”),instructs the IP transmitting and receiving means 10 s as to transferthe message M to the DHCPv4 server 4, supplies the message M thereto,and after that, waits for a reply for a predetermined time. The IPtransmitting and receiving means 10 transmits the message M to theDHCPv4 server 4 via the IPv4 network.

The DHCPv4 server 4 which received the message M gives the IPv4 addressto the IPv6 address (1::1) of the IPv6 terminals 5 (it is now assumedthat 133.144.95.1 was given) and returns an IPv4 address given message(hereinafter, referred to as a “message N”).

The IP transmitting and receiving means 10 which received the message Ntransmits the message N to the IPv4 address capturing means 14. The IPv4address capturing means 14 obtains the IPv4 address (133.144.95.1) fromthe message N and notifies the IP header converting means 11. The IPheader converting means 11 transmits the IPv4 address (133.144.95.1)captured from the DHCPv4 server 4 to the IP address conversioninformation holding means 15 so as to correspond to the IPv6 address(1::1) of the IPv6 terminal 5. The IP address conversion informationholding means 15 forms a new entry into the IP address conversion table151 and registers the correspondence between the IPv4 address(133.144.95.1) and the IPv6 address (1::1) of the IPv6 terminal 5. Theentry of the IP address conversion table 151 is deleted when it is notreferred to for a predetermined time. The IPv4 address (133.144.95.1) isreturned to the DHCPv4 server 4.

When a notification that the corresponding entry has been registered inthe IPv6 address (1::1) of the IPv6 terminal 5 is received or when theregistration of the corresponding entry into the IPv6 address (1::1) ofthe IPv6 terminal 5 is completed, the IP header converting means 11 setsthe IPv4 address (133.144.95.1) into the IPv4 source address field ofthe IPv4 packet. The IPv4 packet formed by performing the IP headerconversion as mentioned above is sent to the IP transmitting andreceiving means 10. The IP transmitting and receiving means 10 transmitsthe IPv4 packet to the IPv4 terminal 2 through the IPv4 network.

In this manner, the communication from the IPv6 terminal 5 to the IPv4terminal 2 can be started.

FIGS. 12 and 13 are flowcharts for operation in another case of startinga communication from the IPv4 terminal 2 to the IPv6 terminal 5. Thiscase has a feature in that “IPv4-mapped IPv6 address” is not used.

Although the IPv4 terminal 2 knows a domain name of the IPv6 terminal 5,it doesn't know an IP address. As shown in FIG. 12, therefore, after theIPv4 terminal 2 transmitted an inquiry message (hereinafter, called a‘message A’) of the IP address corresponding to the domain name of theIPv6 terminal 5 to the IPv4-IPv6 converting apparatus 1 a via the IPv4network 104, it waits for a response for a predetermined time.

The IP transmitting and receiving means 10 of the IPv4-Ipv6 convertingapparatus 1 a receives and processes the message A and sends it to theDNS substituting means 13. The DNS substituting means 13 instructs theIP transmitting and receiving means 10 so as to transfer the message Ato the DNSv4 server 3, sends the message A thereto, and after that,waits for a response for a predetermined time. The IP transmitting andreceiving means 10 transfers the message A to the DNSv4 server 3 via theIPv4 network.

The DNSv4 server 3 which received the message A retrieves the domaininformation of the IPv6 terminal 5. When it is not registered, however,a response to the message A is not returned.

The DNS substituting means 13 which didn't receive the response from theDNSv4 server 3 even after the elapse of a predetermined time instructsthe IP transmitting and receiving means 10 so as to transfer the messageA to the DNSv6 server 6, sends the message A thereto, and after that,waits for a response for a predetermined time. The IP transmitting andreceiving means 10 transfers the message A to the DNSv6 server 6 via theIPv6 network.

The DNSv6 server 6 which received the message A retrieves the domaininformation of the IPv6 terminal 5 and returns the registered domaininformation of the IPv6 terminal 5 to the IPv4-IPv6 converting apparatus1 a as a response message (hereinafter, referred to as a ‘message B’)for the message A.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 a which received the response from the DNSv6 server 6 withinthe predetermined time sends the received message B to the DNSsubstituting means 13. The DNS substituting means 13 obtains the IPv6address (1::1) corresponding to the domain name of the IPv6 terminal 5from the message B and sends it to the IP address conversion informationholding means 15. The IP address conversion information holding means 15retrieves the IP address conversion table 151 by using the IPv6 address(1::1) of the IPv6 terminal 5 as a key. If the relevant entry has beenregistered, the IPv4 address of the entry is returned to the DNSsubstituting means 13. On the other hand, when the relevant entry is notregistered yet, such a fact is notified to the DNS substituting means13.

The DNS substituting means 13 which received the IPv4 address from theIp address conversion information holding means 15 forms a message(hereinafter, referred to as a ‘message E’) to notify of the IPv4address as an IPv4 address corresponding to the domain name of the IPv6terminal 5 and sends it to the IP transmitting and receiving means 10.The IP transmitting and receiving means 10 transmits the message E tothe IPv4 terminal 2 via the IPv4 network.

On the other hand, the DNS substituting means 13 which received thenotification indicating that the entry is not registered yet from the IPaddress conversion information holding means 15 instructs the IPv4address capturing means 14 so as to capture the IPv4 address and, afterthat, waits for a response for a predetermined time. The IPv4 addresscapturing means 14 forms an IPv4 address capturing message (hereinafter,called a ‘message C’) instructs the IP transmitting and receiving means10 so as to transfer the message C to the DHCPv4 server 4, sends themessage C thereto, and after that, waits for a response for apredetermined time. The IP transmitting and receiving means 10 transmitsthe message C to the DHCPv4 server 4 via the IPv4 network.

The DHCPv4 serve 4 which received the message C gives the IPv4 addressto the IPv6 address (1::1) of the IPv6 terminal 5 (it is now assumedthat “133.144.95.1” has been given) and returns an IPv4 address givenmessage (hereinafter, called a ‘message D’).

The IP transmitting and receiving means 10 which received the message Dsends the message D to the IPv4 address capturing means 14. The IPv4address capturing means 14 obtains the IPv4 address (133.144.95.1) fromthe message D and notifies the DNS substituting means 13. The DNSsubstituting means 13 forms the message E to notify of the IPv4 address(133.144.95.1) as an IP address corresponding to the domain name of theIPv6 terminal 5 and sends it to the IP transmitting and receiving means10. The IP transmitting and receiving means 10 transmits the message Eto the IPv4 terminal 2 via the IPv4 network. The DNS substituting means13 sends the IPv4 address (133.144.95.1) captured from the DHCPv4 server4 to the IP address conversion information holding means 15 incorrespondence to the IPv6 address (1::1) of the IPv6 terminal 5. The IPaddress conversion information holding means 15 forms a new entry intothe IP address conversion table 151 and registers a correspondence ofthe IPv4 address (133.144.95.1) and the IPv6 address (1::1) of the IPv6terminal 5. The entry of the IP address conversion table 151 is deletedin the case where it is not referred for a predetermined time. The IPv4address (133.144.95.1) is returned to the DHCPv4 server 4.

As shown in FIG. 13, the IPv4 terminal 2 transmits the IPv4 packet inwhich the IPv4 address (133.144.95.1) notified from the IPv4-IPv6converting apparatus 1 a by the message E has been set into an IPv4destination address field of the IPv4 packet and an own IPv4 address(133.144.95.101) has been set into an IPv4 source address field to theIPv4-IPv6 converting apparatus 1 a through the IPv4 network.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 a sends the received IPv4 packet to the IP header convertingmeans 11. The IP header converting means 11 extracts the IPv4 address(133.144.95.101) from the IPv4 source address field of the IPv4 packet,and sends it to the IP address conversion information holding means 15.The IP address conversion information holding means 15 retrieves the IPaddress conversion table 151 by using the IPv4 address (133.144.95.101)of the IPv4 terminal 2 as a key. When the relevant entry has beenregistered, the IPv6 address of the entry is returned to the IP headerconverting means 11. On the other hand, if the relevant entry is notregistered yet, such a fact is notified to the IP header convertingmeans 11.

The IP header converting means 11 which received the notification thatthe entry is not registered from the IP address conversion informationholding means 15 instructs the IPv6 address capturing means 20 so as tocapture the IPv6 address and, after that, waits for a reply for apredetermined time. The IPv6 address capturing means 20 forms an IPv6address capturing message (hereinafter, referred to as a “message C6”),instructs the IP transmitting and receiving means 10 so as to transferthe message C6 to the DHCPv6 server 7 (FIG. 11), supplies the message C6thereto, and after that, waits a replay for a predetermined time. The IPtransmitting and receiving means 10 transmits the message C6 to theDHCPv6 server 7 via the IPv6 network.

The DHCPv6 server 7 which received the message C6 gives the IPv6 addressto the IPv4 address (133.144.95.101) of the IPv4 terminal 2 (it is nowassumed that 1::101 was given) and returns an IPv6 address given message(hereinafter, referred to as a “message D6”).

The IP transmitting and receiving means 10 which received the message D6supplies the message D6 to the IPv6 address capturing means 20 (FIG.10). The IPv6 address capturing means 20 obtains an IPv6 address(1::101) from the message D6 and notifies the IP header converting means11.

The IP header converting means 11 transmits the IPv6 address (1::101)captured from the DHCPv6 server 7 to the IP address conversioninformation holding means 15 so as to correspond to the IPv4 address(133.144.95.101) of the IPv4 terminal 2. The IP address conversioninformation holding means 15 forms a new entry into the IP addressconversion table 151 and registers a correspondence between the IPv6address (1::101) and the IPv4 address (133.144.95.101) of the IPv4terminal 2. The entry in the IP address conversion table 151 is deletedwhen it is not referred to for a predetermined time. The IPv6 address(1::101) is returned to the DHCPv6 server 7.

When a notification that the relevant entry has been registered in theIPv4 address (133.144.95.101) of the IPv4 terminal 2 is received or whenthe registration of the relevant entry into the IPv4 address(133.144.95.101) of the IPv4 terminal 2 is completed, the IPv6 address(1::101) of the relevant entry is set into an IPv6 transmission addressfield of the IPv6 packet. The IP header converting means 11 extracts theIPv4 address (133.144.95.1) from the IPv4 destination address field ofthe IPv4 packet and sends it to the IP address conversion informationholding means 15. The IP address conversion information holding means 15obtains the IPv6 address (1::1) corresponding to the IPv4 address(133.144.95.1) with reference to the IP address conversion table 151 andsends it to the IP header converting means 11. The IP header convertingmeans 11 sets the IPv6 address (1::1) into an IPv6 destination addressfield of the IPv6 packet. The IPv6 packet formed by performing the IPheader conversion as mentioned above is sent to the IP transmitting andreceiving means 10. The IP transmitting and receiving means 10 transmitsthe IPv6 packet to the IPv6 terminal 5 through the IPv6 network.

In this manner, the communication from the IPv4 terminal 2 to he IPv6terminal 5 can be started.

FIGS. 14 and 15 are flowcharts for operation in another case of startinga communication from the IPv6 terminal 5 to the IPv4 terminal 2. Thiscase has the feature in that the “IPv4-mapped IPv6 address” is not used.

Although the IPv6 terminal 5 knows a domain name of the IPv4 terminal 2,it doesn't know the IP address. As shown in FIG. 14, therefore, the IPv6terminal 5 transmits an inquiry message (hereinafter, called a ‘messageK’) of an IP address corresponding to the domain name of the IPv4terminal 2 to the IPv4-IPv6 converting apparatus 1 a via the IPv6network 106 and, after that, waits for a response for a predeterminedtime.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 a receives and processes the message K and sends it to theDNS substituting means 13. The DNS substituting means 13 instructs theIP transmitting and receiving means 10 so as to transfer the message Kto the DNSv6 server 6, sends the message K thereto, and after that,waits for a response for a predetermined time. The IP transmitting andreceiving means 10 transfers the message K to the DNSv6 server 6 via theIPv6 network.

The DNSv6 server 6 which received the message K retrieves the domaininformation of the IPv4 terminal 2. However, when it is not registered,a response to the message K is not returned.

The DNS substituting means 13 which didn't receive the response from theDNSv6 server 6 even after the elapse of a predetermined time instructsthe IP transmitting and receiving means 10 so as to transfer the messageK to the DNSv4 server 3, sends the message K thereto, and after that,waits for a response for a predetermined time. The IP transmitting andreceiving means 10 transfers the message K to the DNSv4 server 3 via theIPv4 network.

The DNSv4 server 3 which received the message K retrieves the domaininformation of the IPv4 terminal 2 and returns the registered domaininformation of the IPv4 terminal 2 to the IPv4-IPv6 converting apparatus1 a as a response message (hereinafter, called a ‘message L’) for themessage K.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 a which received the response from the DNSv4 server 3 withinthe predetermined time sends the received message L to the DNSsubstituting means 13. The DNS substituting means 13 obtains the IPv4address (133.144.95.101) corresponding to the domain name of the IPv4terminal 2 from the message L.

Subsequently, the DNS substituting means 13 sends the IPv4 address(133.144.95.101) of the IPv4 terminal 2 to the IP address conversioninformation holding means 15. The IP address conversion informationholding means 15 retrieves the IP address conversion table 151 by usingthe IPv4 address (133.144.95.101) of the IPv4 terminal 2 as a key andnotifies the DNS substituting means 13 of information about whether therelevant entry has been registered or not.

The DNS substituting means 13 which received a notification indicatingthat the relevant entry is not registered yet in the IPv4 address(133.144.95.101) of the IPv4 terminal 2 instructs the IPv6 addresscapturing means 20 so as to capture the IPv6 address and, after that,waits for a response for a predetermined time. The IPv6 addresscapturing means 20 forms an IPv6 address capture message (hereinafter,called a ‘message M6’), instructs the IP transmitting and receivingmeans 10 so as to transfer the message M to the DHCPv6 server 7, sendsthe message M thereto, and after that, waits for a response for apredetermined time. The IP transmitting and receiving means 10 transmitsthe message M6 to the DHCPv6 server 7 via the IPv6 network.

The DHCPv6 server 7 which received the message M6 gives the IPv6 addressto the IPv4 address (133.144.95.101) of the IPv4 terminal 2 (it is nowassumed that “1::101” is given) and returns an IPv6 address givenmessage (hereinafter, called a ‘message N6’).

The IP transmitting and receiving means 10 which received the message N6sends the message N6 to the IPv6 address capturing means 20. The IPv6address capturing means 20 obtains the IPv6 address (1::101) from themessage N6 and notifies the DNS substituting means 13. The DNSsubstituting means 13 sends the IPv6 address (1:101) captured from theDHCPv6 server 7 to the IP address conversion information holding means15 in correspondence to the IPv4 address (133.144.95.101) of the IPv4terminal 2. The IP address conversion information holding means 15 formsa new entry into the IP address conversion table 151 and registers acorrespondence of the IPv6 address (1::101) and the IPv4 address(133.144.95.101) of the IPv4 terminal 2. The entry in the IP addressconversion table 151 is deleted when it is not referred to for apredetermined time and the IPv6 address (1::101) is returned to theDHCPv6 server 7.

When a notification indicating that the relevant entry has beenregistered in the IPv4 address (133.144.95.101) of the IPv4 terminal 2is received or when the registration of the relevant entry into the IPv4address (133.144.95.101) of the IPv4 terminal 2 is completed, the DNSsubstituting means 13 forms a message (hereinafter, called a ‘messageP’) to notify of the IPv6 address (1::101) corresponding to the IPv4address (133.144.95.101) of the IPv4 terminal 2 as an IP addresscorresponding to the domain name of the IPv4 terminal 2. The DNSsubstituting means 13 instructs the IP transmitting and receiving means10 so as to transfer the message P to the IPv6 terminal 5 and sends themessage P thereto. The IP transmitting and receiving means 10 transmitsthe message P to the IPv6 terminal 5 via the IPv6 network.

As shown in FIG. 15, the IPv6 terminal 5 transmits the IPv6 packet inwhich the IPv6 address (1::101) notified from the IPv4-IPv6 convertingapparatus 1 a has been set into the IPv6 destination address field ofthe IPv6 packet and the own IPv6 address (1::1) has been set into theIPv6 source address field to the IPv4-IPv6 converting apparatus 1 athrough the IPv6 network.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 a sends the received IPv6 packet to the IP header convertingmeans 11. The IP header converting means 11 extracts the IPv6 address(1::101) from the IPv6 destination address field of the Ipv6 packet, andsupplies it to the IP address conversion information holding means 15.The IP address conversion information holding means 15 obtains the IPv4address (133.144.95.101) corresponding to the IPv6 address (1::101) withreference to the IP address conversion table 151 and transmits to the IPheader converting means 11. The IP header converting means 11 sets theIPv4 address (133.144.95.101) into the IPv4 destination address field ofthe IPv4 packet. The IP header converting means 11 extracts the IPv6address (1::1) from the IPv6 source address field of the IPv6 packet andsends it to the IP address conversion information holding means 15.

The IP address conversion information holding means 15 retrieves the IPaddress conversion table 151 by using the IPv6 address (1::1) of theIPv6 terminal 5 as a key and notifies the IP header converting means 11of a result indicating whether the relevant entry has been registered ornot.

The IP header converting means 11 which received the notification thatthe relevant entry is not registered yet in the IPv6 address (1::1) ofthe IPv6 terminal 5 instructs the IPv4 address capturing means 14 so asto capture the Ipv4 address and, after that, waits for a reply for apredetermined time. The IPv4 address capturing means 14 forms an IPv4address capturing message (hereinafter, referred to as a “message M”),instructs the IP transmitting and receiving means 10 so as to transferthe message M to the DHCPv4 server 4, supplies the message M thereto,and after that, waits for a reply for a predetermined time. The IPtransmitting and receiving means 10 transmits the message M to theDHCPv4 server 4 via the IPv4 network.

The DHCPv4 server 4 which received the message M gives the IPv4 addressto the IPv6 address (1::1) of the IPv6 terminal 5 (it is now assumedthat 133.144.95.1 was given) and returns an IPv4 address given message(hereinafter, referred to as a “message N”).

The IP transmitting and receiving means 10 which received the message Ntransmits the message N to the IPv4 address capturing means 14. The IPv4address capturing means 14 obtains the IPv4 address (133.144.95.1) fromthe message N and notifies the IP header converting means 11. The IPheader converting means 11 transmits the IPv4 address (133.144.95.1)captured from the DHCPv4 server 4 to the IP address conversioninformation holding means 15 so as to correspond to the IPv6 address(1::1) of the IPv6 terminal 5. The IP address conversion informationholding means 15 forms a new entry into the IP address conversion table151 and registers the correspondence between the IPv4 address(133.144.95.1) and the IPv6 address (1::1) of the IPv6 terminal 5. Theentry of the IP address conversion table 151 is deleted when it is notreferred to for a predetermined time. The IPv4 address (133.144.95.1) isreturned to the DHCPv4 server 4.

When a notification that the corresponding entry has been registered inthe IPv6 address (1::1) of the IPv6 terminal 5 is received or when theregistration of the corresponding entry into the IPv6 address (1::1) ofthe IPv6 terminal 5 is completed, the IP header converting means 11 setsthe IPv4 address (133.144.95.1) into the IPv4 source address field ofthe IPv4 packet. The IPv4 packet formed by performing the IP headerconversion as mentioned above is sent to the IP transmitting andreceiving means 10. The IP transmitting and receiving means 10 transmitsthe IPv4 packet to the IPv4 terminal 2 through the Ipv4 network.

In this manner, the communication from the IPv6 terminal 5 to the IPv4terminal 2 can be started.

(B) Second Embodiment

FIG. 16 is a constructional diagram of an IPv4-IPv6 converting apparatus1 b according to the second embodiment of the invention.

The IPv4-IPv6 converting apparatus 1 b has a construction obtained byadding IPv4 address pool means 16 to the IPv4-IPv6 converting apparatus1 a of FIG. 1.

At the time of an activation or the like of the IPv4-IPv6 convertingapparatus 1 b, the IPv4 address capturing means 14 captures a pluralityof IPv4 addresses in a lump from the DHCPv4 server 4 and pools them intothe IPv4 address pool means 16. At the start of the communicationbetween the IPv4 terminal 2 and IPv6 terminal 5, the IPv4 addresscapturing means 14 obtains the IPv4 addresses from the IPv4 address poolmeans 16.

By using the IPv4-IPv6 converting apparatus 1 b, a load of the IPv4network 104 can be reduced more than the case of capturing the IPv4addresses one by one from the DHCPv4 server 4.

(C) Third Embodiment

FIG. 17 is a constructional diagram of an IPv4-IPv6 converting apparatus1 c according to the third embodiment of the invention.

The IPv4-IPv6 converting apparatus 1 c has a construction obtained byproviding DHCPv4 server means 17 in place of the IPv4 address capturingmeans 14 of the IPv4-IPv6 converting apparatus 1 a of FIG. 1.

By using the IPv4-IPv6 converting apparatus 1 c, there is no need toindividually provide the DHCPv4 server 4.

(D) Fourth Embodiment

FIG. 18 is a constructional diagram of an IPv4-IPv6 converting apparatus1 d according to the fourth embodiment of the invention.

The IPv4-IPv6 converting apparatus 1 d has a construction obtained byadding DNSv4 server means 18 and DNSv6 server means 19 to the IPv4-IPv6converting apparatus 1 a of FIG. 1.

By using the IPv4-IPv6 converting apparatus 1 d, there is no need toindividually provide the DNSv4 server 3 and DNSv6 server 6.

(E) Fifth Embodiment

The fifth embodiment will now be described by using FIGS. 19, 20, and21.

FIG. 19 is a diagram of a communication network system 108 in which theIPv6 network 106 and an IPv6 network 107 are connected through the IPv4network 104. The communication network system 108 has a constructionobtained by adding the IPv6 network 107 to the communication networksystem 100 of FIG. 3. The IPv6 network 107 is connected to the IPv4network 104 by an IPv4-IPv6 converting apparatus 111.

In addition to an IPv6 terminal 115, a DNSv6 server 116 for managingdomain information in the IPv6 network 107 is connected to the IPv6network 107.

It is now assumed that (2::1) has been allocated as an IPv6 address tothe IPv6 terminal 115. In addition to 100 IPv4 addresses of“133.144.95.1” to “133.144.95.100”, 100 IPv4 addresses of “133.144.96.1”to “133.144.96.100” have been pooled in the DHCPv4 server 4. It is alsoassumed that the IPv4-IPv6 converting apparatus 111 has been set as aDNS server in the IPv6 terminal 115. It is assumed that the DNSv6 server116 has been set as a DNS server and the DHCPv4 server 4 has been set asa DHCP server into the IPv4-IPv6 converting apparatus 111. It is alsoassumed that the IPv4-IPv6 converting apparatus 111 has been set as aDNS server of an upper domain into an IPv4-IPv6 converting apparatus 1.

FIGS. 20 and 21 are flowcharts for the operation in case of starting acommunication from the IPv6 terminal 5 to the IPv6 terminal 115.

Although the IPv6 terminal 5 knows a domain name of the IPv6 terminal115, it doesn't know an IP address. Therefore, as shown in FIG. 20, theIPv6 terminal 5 transmits an inquiry message (hereinafter, called a‘message Q’) of the IP address corresponding to the domain name of theIPv6 terminal 115 to the IPv4-IPv6 converting apparatus 1 via the IPv6network 106 and, after that, waits for a response for a predeterminedtime.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 receives and processes the message Q and sends it to the DNSsubstituting means 13. The DNS substituting means 13 instructs the IPtransmitting and receiving means 10 so as to transfer the message Q tothe IPv4-IPv6 converting apparatus 111, sends the message Q thereto, andafter that, waits for a response for a predetermined time. The IPtransmitting and receiving means 10 transfers the message Q to theIPv4-IPv6 converting apparatus 111 via the IPv4 network.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 111 receives and processes the message Q and sends it to theDNS substituting means 13. The DNS substituting means 13 instructs theIP transmitting and receiving means 10 so as to transfer the message Qto the DNSv6 server 116, sends the message Q thereto, and after that,waits for a response for a predetermined time. The IP transmitting andreceiving means 10 transfers the message Q to the DNSv6 server 116 viathe IPv6 network.

The DNSv6 server 116 which received the message Q retrieves domaininformation of the IPv6 terminal 115 and returns the registered domaininformation of the IPv6 terminal 115 to the IPv4-IPv6 convertingapparatus 111 as a response message (hereinafter, called a ‘message R’)for the message Q.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 111 which received the response from the DNSv6 server 116within a predetermined time sends the received message R to the DNSsubstituting means 13. The DNS substituting means 13 obtains an IPv6address (2::1) corresponding to the domain name of the IPv6 terminal 115from the message R and sends it to the IP address conversion informationholding means 15. The IP address conversion information holding means 15retrieves the IP address conversion table 151 by using the IPv6 address(2::1) of the IPv6 terminal 115 as a key. If the relevant entry has beenregistered, the holding means 15 returns the IPv4 address of the entryto the DNS substituting means 13. On the other hand, if the relevantentry is not registered yet, such a fact is notified to the DNSsubstituting means 13.

The DNS substituting means 13 which received the IPv4 address from theIP address conversion information holding means 15 forms a message(hereinafter, called a ‘message U’) to notify of the IPv4 address as anIPv4 address corresponding to the domain name of the IPv6 terminal 115and sends the message U to the IP transmitting and receiving means 10.The IP transmitting and receiving means 10 transmits the message U tothe IPv4-IPv6 converting apparatus 1 via the IPv4 network.

The DNS substituting means 13 which received the notification that theentry is not registered yet from the IP address conversion informationholding means 15 instructs the IPv4 address capturing means 14 so as tocapture the IPv4 address and, after that, waits for a response for apredetermined time. The IPv4 address capturing means 14 forms an IPv4address capturing message (hereinafter, called a ‘message S’), instructsthe IP transmitting and receiving means 10 so as to transfer the messageS to the DHCPv4 server 4, sends the message S thereto, and after that,waits for a response for a predetermined time. The IP transmitting andreceiving means 10 transmits the message S to the DHCPv4 server 4 viathe IPv4 network.

The DHCPv4 server 4 which received the message S gives the IPv4 addressto the IPv6 address (2::1) of the IPv6 terminal 115 (it is now assumedthat “133.144.96.1” is given) and returns an IPv4 address given message(hereinafter, called a ‘message T’).

The IP transmitting and receiving means 10 which received the message Tsends the message T to the IPv4 address capturing means 14. The IPv4address capturing means 14 obtains the IPv4 address (133.144.96.1) fromthe message T and notifies the DNS substituting means 13. The DNSsubstituting means 13 forms the message U to notify of the IPv4 address(133.144.96.1) as an IP address corresponding to the domain name of theIPv6 terminal 115 and sends it to the IP transmitting and receivingmeans 10. The IP transmitting and receiving means 10 transmits themessage U to the IPv4-IPv6 converting apparatus 1 via the IPv4 network.The DNS substituting means 13 sends the IPv4 address (133.144.96.1)captured from the DHCPv4 server 4 to the IP address conversioninformation holding means 15 in correspondence to the IPv6 address(2::1) of the IPv6 terminal 115. The IP address conversion informationholding means 15 forms a new entry into the IP address conversion table151 and registers a correspondence of the IPv4 address (133.144.96.1)and the IPv6 address (2::1) of the IPv6 terminal 115. The entry in theIP address conversion table 151 is deleted when it is not referred tofor a predetermined time. The IPv4 address (133.144.96.1) is returned tothe DHCPv4 server 4.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 which received the response from the IPv4-IPv6 convertingapparatus 111 within the predetermined time sends the received message Uto the DNS substituting means 13. The DNS substituting means 13 obtainsthe IPv4 address (133.144.96.1) corresponding to the domain name of theIPv6 terminal 115 from the message U. The DNS substituting means 13forms a message (hereinafter, called a ‘message V’) to notify of an‘IPv4-mapped IPv6 address’ (::FFFF:133.144.96.1) in which the IPv4address (133.144.96.1) derived from the message U has been set as an IPaddress corresponding to the domain name of the IPv6 terminal 115,instructs the IP transmitting and receiving means 10 so as to transferthe message V to the IPv6 terminal 5, and sends the message V thereto.The IP transmitting and receiving means 10 transmits the message V tothe IPv6 terminal 5 via the IPv6 network.

As shown in FIG. 21, the IPv6 terminal 5 transmits the IPv6 packet inwhich the IPv6 address (::FFFF:133.144.96.1) notified from the Ipv4-Ipv6converting apparatus 1 has been set into the IPv6 destination addressfield of the IPv6 packet and the own IPv6 address (1::1) has been setinto the IPv6 source address field to the IPv4-IPv6 converting apparatus1 through the IPv6 network.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 1 sends the received IPv6 packet to the IP header convertingmeans 11. The

IP header converting means 11 extracts the IPv6 address(::FFFF:133.144.96.1) from the IPv6 destination address field of theIPv6 packet, converts the IPv6 address to the IPv4 address(133.144.96.1) of 32 bits by deleting the fixed pattern of 96 bitsmentioned above, and sets the IPv4 address into the IPv4 destinationaddress field of the IPv4 packet. The IP header converting means 11extracts the IPv6 address (1::1) from the IPv6 source address field ofthe IPv6 packet and sends it to the IP address conversion informationholding means 15.

The IP address conversion information holding means 15 retrieves the IPaddress conversion table 151 by using the IPv6 address (1::1) of theIPv6 terminal 5 as a key and notifies the IP header converting means 11of a message indicating whether the relevant entry has been registeredor not.

The IP header converting means 11 which received the notification thatthe relevant entry is not registered yet in the IPv6 address (1::1) ofthe IPv6 terminal 5 instructs the IPv4 address capturing means 14 so asto capture the IPv4 address and, after that, waits for a response for apredetermined time. The IPv4 address capturing means 14 forms an IPv4address capturing message (hereinafter, called a ‘message W’), instructsthe IP transmitting and receiving means 10 so as to transfer the messageW to the DHCPv4 server 4, sends the message W thereto, and after that,waits for a response for a predetermined time. The IP transmitting andreceiving means 10 transmits the message W to the DHCPv4 server 4 viathe IPv4 network.

The DHCPv4 server 4 which received the message W gives the IPv4 addressto the IPv6 address (1::1) of the IPv6 terminal 5 (it is now assumedthat “133.144.95.1” is given) and returns an IPv4 address given message(hereinafter, called a ‘message X’).

The IP transmitting and receiving means 10 which received the message Xsends the message X to the IPv4 address capturing means 14. The IPv4address capturing means 14 obtains the IPv4 address (133.144.95.1) fromthe message X and notifies the IP header converting means 11. The IPheader converting means 11 sends the IPv4 address (133.144.95.1)captured from the DHCPv4 server 4 to the IP address conversioninformation holding means 15 in correspondence to the IPv6 address(1::1) of the IPv6 terminal 5. The IP address conversion informationholding means 15 forms a new entry into the IP address conversion table151 and registers a correspondence of the IPv4 address (133.144.95.1)and the IPv6 address (1::1) of the IPv6 terminal 5. The entry in the IPaddress conversion table 151 is deleted when it is not referred to for apredetermined time. The IPv4 address (133.144.95.1) is returned to theDHCPv4 server 4.

When a notification that the relevant entry has been registered in theIpv6 address (1::1) of the IPv6 terminal 5 is received or when theregistration of the relevant entry into the IPv6 address (1::1) of theIPv6 terminal 5 is completed, the IP header converting means 11 sets theIPv4 address (133.144.95.1) into the IPv4 source address field of theIPv4 packet. The IPv4 packet formed by performing the IP headerconversion as mentioned above is sent to the IP transmitting andreceiving means 10. The IP transmitting and receiving means 10 transmitsthe IPv4 packet to the IPv4-IPv6 converting apparatus 111 through theIPv4 network.

The IP transmitting and receiving means 10 of the IPv4-IPv6 convertingapparatus 111 sends the received IPv4 packet to the IP header convertingmeans 11. The IP header converting means 11 extracts the IPv4 address(133.144.95.1) from the IPv4 source address field of the IPv4 packet,converts the IPv4 address to an ‘IPv4-mapped IPv6 address’(::FFFF:133.144.95.1) of 128 bits by adding the fixed pattern of 96 bitsmentioned above, and sets it into the IPv6 source address field of theIPv6 packet. The IP header converting means 11 extracts the IPv4 address(133.144.96.1) from the IPv4 destination address field of the IPv4packet and sends the IPv4 address to the IP address conversioninformation holding means 15. The IP address conversion informationholding means 15 obtains the IPv6 address (2::1) corresponding to theIPv4 address (133.144.96.1) with reference to the IP address conversiontable 151 and sends the IPv6 address to the IP header converting means11. The IP header converting means 11 sets the IPv6 address (2::1) intothe IPv6 destination address field of the IPv6 packet. The IPv6 packetformed by performing the IP header conversion as mentioned above is sentto the IP transmitting and receiving means 10. The IP transmitting andreceiving means 10 transmits the IPv6 packet to the IPv6 terminal 115through the IPv6 network.

In this manner, the communication from the IPv6 terminal 5 to the IPv6terminal 115 can be started.

Although the invention has been described above by using thecommunication between the two IPv6 terminals when there is the IPv4network on the communication path, a similar construction can be alsoapplied to a communication between the two IPv4 terminals when there isthe IPv6 network on the communication path.

Although the invention has been also described above by using thecommunication between the two IPv6 terminals when there is only one IPv4network on the communication path, a similar construction can be alsoapplied to a communication between the two IPv6 terminals when there area plurality of IPv4 networks and IPv6 networks on the communicationpath, a communication between the two IPv4 terminals, and acommunication between the IPv4 terminal and the IPv6 terminal.

According to the IPv4-IPv6 communicating method and IPv4-IPv6 convertingapparatus of the invention, only the IPv6 protocol is installed to theIPv6 terminal and even if the IPv4 address is not previously fixedlyallocated, the communication between the IPv4 terminal and the IPv6terminal can be executed. Further, since the partner can be designatednot by the actual IP address but by the domain name, the communicationcan be performed without mutually being aware whether the partner usesthe IPv4 or IPv6 as a communication protocol.

As for the registration and deletion of the entry in the conversiontable of the IPv4 address and IPv6 address, by limiting by the settingof a manager, the system can also become a fire wall between the IPv4network and the IPv6 network.

A communication between the two IPv6 terminals partitioned by the IPv4network can be also executed without preliminarily performing anyspecial setting. Similarly, a communication between the two IPv4terminals partitioned by the IPv6 network can be executed withoutpreliminarily performing any special setting.

Further, even if any special setting is not preliminarily performed, acommunication between the IPv4 terminals partitioned by a plurality ofIPv4 networks and IPv6 networks, a communication between the IPv6terminals, and a communication between the IPv4 terminal and the IPv6terminal can be executed.

1. A communication apparatus which performs communication between afirst apparatus using IPv4 and a second apparatus using IPv6,comprising: an IP transmitter/receiver which transmits a message to thefirst apparatus and receives the message from the first apparatus; anaddress inquiry unit, in response to receive a first message forinquiring an IP address of the second apparatus from the firstapparatus, which transmits a second message for inquiring said IPaddress of the second apparatus to an address manager apparatus via theIP transmitter/receiver; and a holding unit which holds a correspondencebetween an IPv4 address and an IPv6 address, wherein, if the addressinquiry unit receives a third message including a first IPv6 address ofthe second apparatus from the manager apparatus, the address inquiryunit allocates a first IPv4 address to said first IPv6 address; andwherein the holding unit holds a correspondence between said first IPv6address and said first IPv4 address.
 2. The communication apparatusaccording to claim 1, wherein the address inquiry unit transmits afourth message including said first IPv4 address to the first apparatusvia the IP transmitter/receiver.
 3. The communication apparatusaccording to claim 1, wherein the address inquiry unit transmits thesecond message to the manager apparatus which manages said IPv6 address.4. The communication apparatus according to claim 1, wherein the addressinquiry unit transmits the second message to the manager apparatus whichmanages said IPv4 address and said IPv6 address.
 5. The communicationapparatus according to claim 1, wherein the address inquiry unittransmits the second message to a first manager apparatus which managessaid IPv4 address and a second manager apparatus which manages said IPv6address.
 6. The communication apparatus according to claim 5, whereinthe IP transmitter/receiver transmits the second message to the secondmanager apparatus if the IP transmitter/receiver does not receive thethird message from the first manager apparatus.
 7. The communicationapparatus according to claim 1, wherein the manager apparatus is a DNSapparatus.
 8. The communication apparatus according to claim 1, furthercomprising an address conversion unit, wherein, if the addressconversion unit receives a packet including said first IPv4 address as adestination address from the first apparatus, the address conversionunit obtains the first IPv6 address corresponding to said first IPv4address from the holding unit, converts the destination address of thepacket into the obtained first IPv6 address, transmits a packetincluding the converted first IPv6 address to the second apparatus viathe IP transmitter/receiver.
 9. A communication apparatus which performscommunication between a first apparatus using IPv6 and a secondapparatus using IPv4, comprising: an IP transmitter/receiver whichtransmits a message to the first apparatus and receives the message fromthe first apparatus; an address inquiry unit, in response to receive afirst message for inquiring an IP address of the second apparatus fromthe first apparatus, which transmits a second message for inquiring saidIP address of the second apparatus to an address manager apparatus viathe IP transmitter/receiver; and a holding unit which holds acorrespondence between an IPv4 address and an IPv6 address, wherein, ifthe address inquiry unit receives a third message including a first IPv4address of the second apparatus from the manager apparatus, the addressinquiry unit allocates a first IPv6 address to said first IPv4 address;and wherein the holding unit holds a correspondence between said firstIPv4 address and said first IPv6 address.
 10. The communicationapparatus according to claim 9, wherein the address inquiry unittransmits a fourth message including said first IPv6 address to thefirst apparatus via the IP transmitter/receiver.
 11. The communicationapparatus according to claim 9, wherein the address inquiry unittransmits the second message to the manager apparatus which manages saidIPv4 address.
 12. The communication apparatus according to claim 9,wherein the address inquiry unit transmits the second message to themanager apparatus which manages said IPv4 address and said IPv6 address.13. The communication apparatus according to claim 9, wherein theaddress inquiry unit transmits the second message to a first managerapparatus which manages said IPv4 address and a second manager apparatuswhich manages said IPv6 address.
 14. The communication apparatusaccording to claim 13, wherein the IP transmitter/receiver transmits thesecond message to the first manager apparatus if the IPtransmitter/receiver does not receive the third message from the secondmanager apparatus.
 15. The communication apparatus according to claim 9,wherein the manager apparatus is a DNS apparatus.
 16. The communicationapparatus according to claim 9, further comprising an address conversionunit, wherein, if the address conversion unit receives a packetincluding said first IPv6 address as a destination address from thefirst apparatus, the address conversion unit obtains said first IPv4address corresponding to said first IPv6 address from the holding unit,converts the destination address of the packet into the obtained firstIPv4 address, transmits a packet including the converted first IPv4address to the second apparatus via the IP transmitter/receiver.